1. Field of the Invention
The present invention relates to a mercury removing apparatus and a mercury removing method.
Priority is claimed on Japanese Patent Application No. 2007-335475, filed Dec. 27, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
Coal combustion gas includes a very small amount of mercury caused by coal. In this mercury, there is poorly-soluble metal mercury (Hg0), water-soluble dyadic mercury (Hg2+(HgCl2)), and granular mercury (HgP) attached to combustion ashes.
In an exhaust gas processing system, most granular mercury (HgP) is removed by a precipitation device such as a bug filter or an electrostatic precipitator (EP), and dyadic mercury (Hg2+) can be removed by a wet flue gas desulfurization (FGD) device with high efficiency. However, it is difficult to remove metal mercury (Hg0) by EP and FGD. In this case, a method has been proposed for removing metal mercury (Hg0) by oxidizing metal mercury (Hg0) into water-soluble dyadic mercury (Hg2+).
For example, in Japanese Patent Application Laid-Open No. 10-230137, since a concentration of hydrogen chloride (HCl) in exhaust gas is as small as 1 ppm to several tens ppm, mercury oxidizing efficiency is low and mercury trapping efficiency in EP or FGD is low.
To raise the concentration of hydrogen chloride in exhaust gas, a method is described for raising a concentration of hydrogen chloride in exhaust gas by injecting a material having halogen such as a chlorine compound to a passage of exhaust gas. In Japanese Patent Application Laid-Open No. 2000-325746, a method is described for burning a chlorine compound together with coal.
Thus, it is possible to efficiently oxidize metal mercury (Hg0), and it is possible to remove metal mercury (Hg0) from exhaust gas.
However, in such a mercury removing method, since a separate chemical such as a chlorine compound is necessary, costs for processing exhaust gas increases.
In consideration of such a problem, the invention provides a mercury removing apparatus and a mercury removing method to improve a mercury removing ratio with low cost.